1. Field of the Invention
The present invention relates generally to an arrangement for maintaining a clear air zone in a high pressure, high temperature steam vulcanization process, and more particularly to a method and apparatus for maintaining a clear air zone for permitting optical measurements in a high pressure, high temperature steam continuous vulcanization chamber.
2. Description of the Prior Art:
In the cable manufacturing industry one of the critical requirements is that the cable being produced is of the proper size. Unfortunately, accurate sizing measurements are particularly complicated when cable is being prepared according to conventional continuous vulcanization processes. In such processes the cable, with an appropriate layer of unvulcanized insulation, is produced in an extruding machine, and is subsequently fed into a tubular continuous vulcanization chamber of perhaps 400 to 500 ft. in length filled with high pressure, high temperature steam. The insulation material coating the cable is vulcanized as the extruded cable passes through this long vulcanization chamber.
A problem that exists in this environment is that of measuring the cable size during the vulcanization process. In the past, measurements of cable size have been conducted with sizing dies as the cable emerges from the long vulcanization tube. This measuring technique is very inefficient, however, because if the cable is found to be off size, a minimum of 400 ft. (i.e. the length of the vulcanization chamber) of scrap cable must be manufactured before appropriate changes can be made in the extruding process to try to bring the cable to size. Another 400 ft. of cable must then be produced before the effect of these changes can be evaluated. Furthermore, it may take as long as one hour for the cable to emerge from the vulcanization tube, causing significant delay times in adjustment of the extruding operation not to mention costly material losses when off-size cable is being produced. Thus simply waiting until the cable emerges from the end of the vulcanization tube results in a very costly and inefficient technique of monitoring cable size.
Ideally, the size of a cable should be measured as soon as it emerges from the extruding machines and enters the vulcanization tube. However, the high pressure and high temperature existing in the vulcanization tube makes direct access to the tube totally impractical since the vulcanization tube must remain sealed to prevent the steam contained in it from escaping. Optical measurements of the cable through a sealed optical window using a device such as a laser micrometer would appear in theory to provide a solution to the problem of accurately measuring cable diameters. Unfortunately, this solution has not worked satisfactorily in the past because; first, it has been found to be virtually impossible to maintain an optical window into to the vulcanization tube free of condensation which tends to diffuse light, thereby rendering optical measurements unreliable and, second, condensation of steam on the cable itself causes water droplets which interfere with accurate optical measurement of the cable. It is accordingly understood by those skilled in the art that if an optical window could be maintained in a clear and transparent state in the continuous vulcanization system, and if the cable could be kept free of condensation droplets, cable size monitoring techniques could be made highly efficient by the use of laser micrometers at the point where cable emerges from the extruding machine. Such a system, depending upon the maintenance of a clear optical window and a clean cable, would minimize product wastage and at the same time minimize time lost in modifying the extruding process to correct for errors in the dimension of the extruded cable.